Imaging system requires an adjustable aperture in order to regulate the amount of light admitted to the imaging system.
The conventional imaging system uses iris diaphragm devices to adjust the aperture, wherein a plurality of mechanical movable blades are attached to a mount in an imaging system and rotations of the blades make the size of apertures changed. These mechanical iris diaphragm devices present many problems. They tend to suffer from wear and tear by repeated uses and have shock and noise problems. Also, due to its macroscopic moving structure, they draw high power and make the imaging system bulky. Furthermore, the aperture changing speed is low, which can be a critical problem for high frame rate imaging systems or auto-focusing imaging systems having an auto exposure feature.
To resolve these problems caused by the mechanical iris diaphragm, U.S. Pat. No. 3,955,208 to Wick discloses a liquid crystal diaphragm, wherein an aperture size is changed by changing the transparency of at least one portion of one or more bodies of liquid crystal. The liquid crystal diaphragm device, however, has a limited diaphragm changing rate as well as a low contrast rate. Also, this device cannot provide a focusing capability.
While the liquid crystal diaphragm is a transmittive type light modulator, the reflective type light modulator such DMD (Digital Micromirror Device) have been used as light shutters as disclosed U.S. Pat. No. 6,900,901 to Harada. In conventional micromirror devices, each micromirror has bistable states (on and off), wherein when a micromirror is on, the incident light is reflected to a designated area and when the micromirror is off, the incident light is reflected to a light absorber. These micromirror devices are used as a shutter device rather than an iris device. Also, the conventional micromirror device does not have a focusing capability because of its limited motion range.